Monazite, a phosphate of the rare earths, is the principal mineral from which the cerium earths and thorium are obtained. Fluviatile monazite placers were mined in the Piedmont province of North and South Carolina from 1887 to 1911, and again intermittently from 1915 to 1917; but the principal sources In recent years have been the beach placers of India and Brazil. In 1946, an embargo was placed on the exportation of Indian monazite, and the Brazilian production has not increased materially to replace this loss. Accordingly monazite in recent years has become a scarce commodity.
The principal domestic sources from which monazite may be recovered commercially are in Idaho and in the Piedmont province of the southeastern States. Some monazite is now being produced in Idaho, and a small output is being recovered as a byproduct of heavy mineral mining in Florida. The southeastern placers were not exhausted by the earlier mining and new deposits have been discovered; but production from this region awaits adequate exploration.
The country rock of the southeastern Piedmont province is a complex assemblage of metamorphic and igneous rocks. The monazite occurs in two belts.
A western belt has been traced from east-central Virginia for 600 miles southwestward into Alabama; and an eastern belt has been traced from the vicinity of Fredericksburg, Va., south-southwestward for 200 miles into North Carolina. Monazite-bearing rocks near. Rion, S. C., appear to indicate a southwestward continuation of the eastern belt.
The western, or principal belt, includes the placers that were formerly mined in North and South Carolina. These placers were sampled, and the monazite was separated from the best of the samples, for mineralogical and chemical analysis. The tabulated results show a mean tenor, in the headwater placers of highest grade, of 8.4 pounds of monazite to the cubic yard. Farther downstream where mining must be done to obtain larger yardages, the tenor will be much lower. The mean contents of ThO2 and U3O8 in the placer monazite are shown to be respectively about 5. 7 and 0.4 percents.
The western monazite belt was explored northeastward and southwestward from the sites of earlier mining by sampling the weathered bedrock; and the eastern monazite belt was discovered and sampled by the same technique. The principal source-rocks are certain types of granitic intrusives, granitized and pegmatitized country rock, and certain granitic gneisses of the Carolina gneiss. Some of the associated pegmatites also contain high percentages of monazite. Most of the monazite-bearing granitic intrusives are quartz monzonite or closely related rocks. The mean tenor of monazite in bedrock is about 0.006 percent. No search has yet been made for workable placers in these belts beyond the original sites of mining.
Monazite derived from bedrock sources in the piedmont has been found in small quantities in all of the Coastal Plain formations, but the tenor is too low to warrant mining for this mineral alone. At favored localities, however, commercial deposits of heavy minerals may be found, similar to those now being mined in Florida, that may yield monazite as a byproduct. Small fluviatile deposits of heavy minerals, including monazite, that were reconcentrated from detrital deposits of Cretaceous age, have recently been found in Georgia and South Carolina, along the inner margin of the Coastal Plain.
The monazite belts are conceived to be the sites of early pre-Cambrian valleys, wherein detrital monazite derived from an earlier pre-Cambrian granite, was distributed. These ancient fluviatile deposits were later reconstituted into gneisses of Carolina age, and parts of the latter were remelted to form monazite-bearing granitic intrusives. Some of the monazite-bearing granites may also have originated by the remelting of earlier pre-Cambrian intrusives. The distribution of iron ores in the monazite-bearing rocks appears to accord with
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USGS Numbered Series
Monazite deposits of the southeastern Atlantic States